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If you've been waiting for Siri on iPad, I've got bad news for you. It's going to take a bit more time for Siri to fully debut — if at all. In fact, Siri isn't about to launch on the newest iPad at all during its third generation debut.

Yes, you'll see dictation on the new iPad. All the dictation how-to covered in our Talking to Siri book will apply. But you won't be able to tell Siri to wake you up in the morning or ask her if you should take an umbrella to work. That's because Siri isn't just about speech-to-text; Siri's actually about providing an integrated assistant that translates speech commands to operating system (OS) services.

How Siri Works on Your iPhone

Siri on the iPhone offers features that connect to your address book, to your music player, to your alarm clock, and so forth. Each of these is implemented through an expansion of the iOS core services. Core services refer to the fundamental OS user interface on Apple devices. This includes the Finder app on OS X and what's internally called "Springboard" on iOS.

Springboard refers to the icon-launching screen and the recent applications tray that appears when you double-click the Home button. Springboard both lets you launch applications and provides a framework for applications to talk to each other, and it provides the key technology that powers Siri.

When you speak to Siri, the iPhone translates your speech into phonemes. Phonemes are the perceptually distinct units that make up the sound you utter. The Siri system sends these phonemes off to be processed remotely as "Server Bound" requests. After processing at Apple's data center, they return to your iPhone as client bound "Ace" (assistant) commands. These are structured application-specific assistant elements such as reminder payloads for creating new reminders, and calendar events for reporting about your day, phone calls that need placing, and the "user utterance" speech bubbles that show you what you just said.

Apple's highly structured Ace implementation means that the Siri Assistant can process your requests and transform them into meaningful searches and updates for iOS. A reminder is sent off through Springboard to the onboard reminders app. Phone calls are diverted to the Phone app. Audio playback commands are handled by the built-in iPod features of the Music app.

Each Ace element has a specific app target, whether it's internal to the Siri interface (show a Wolfram-Alpha search result, update a speech bubble, or speak Siri's response) or external (the Clock app, Reminders, Mobile Safari, Messages, iCal, etc). Ace commands are routed for handling to the app or core service that best handles it.

And Why She Can't Easily Make the Leap to Your iPad

The iPad lacks many features supported by the current Siri Ace command set. You cannot set timers or place calls and many units do not offer onboard GPS — although they do provide Wi-Fi positioning. Lacking GPS means that in-car directions in particular may not be available; interstate highways are not noted for their static Wi-Fi hotspots, especially outside of major cities.

These missing features mean that for Apple to deploy Siri to the iPad would require a major platform-specific rewrite. That's a lot to ask when you consider that Siri itself remains in its initial beta release.

What's more, Siri provides one of the biggest sales points at this time for Apple's premiere product, the iPhone 4S. Keeping it exclusive to the 4S can only help those sales and won't hurt the new iPad. Extending dictation features to the iPad is sure to be a winner on its own: users will be able to speak emails and Google search terms, as well as dictate tweets and Facebook updates.

I certainly look forward to seeing Siri make the jump to more platforms, but recognizing how fundamental its implementation is to the core OS and its services helps me understand why it will still take a bit of time before we see that happen. When Siri does move fully to the iPad, expect it to provide a limited vocabulary of actions. Without telephony and the alarm app, and without GPS on the WiFi units, Siri's abilities may be curtailed.